In percutaneous transluminal coronary angioplasty (PTCA), a balloon catheter is inserted through a brachial or femoral artery, positioned across a coronary artery occlusion, and inflated to compress the atherosclerotic plaque to open, by remodeling, the lumen of the coronary artery. The balloon is then deflated and withdrawn. Problems with PTCA include formation of dissections, intimal flaps and torn arterial linings, all of which can create another occlusion in the lumen of the coronary artery. Moreover, thrombosis and restenosis may occur several months after the procedure and create a need for additional angioplasty or a surgical bypass operation. Stents are used to address these issues. Stents are small, intricate, implantable medical devices and are generally implanted to stop negative remodeling, reduce occlusions, inhibit thrombosis and restenosis, and maintain patency within vascular lumens such as, for example, the lumen of a coronary artery.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. Stent delivery refers to introducing and transporting the stent through an anatomical lumen to a desired treatment site, such as a lesion in a vessel. An anatomical lumen can be any cavity, duct, of a tubular organ such as a blood vessel, urinary tract, and bile duct. Stent deployment corresponds to expansion of the stent within the anatomical lumen at the region requiring treatment. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into an anatomical lumen, advancing the catheter in the anatomical lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen with the stent remaining at the treatment location.
In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon prior to insertion in an anatomical lumen. At the treatment site within the lumen, the stent is expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn from the stent and the lumen, leaving the stent at the treatment site. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath. When the stent is at the treatment site, the sheath may be withdrawn which allows the stent to self-expand.
Stents are often modified to provide drug delivery capabilities to further address thrombosis and restenosis. Stents may be coated with a polymeric carrier impregnated with a drug or therapeutic substance. A conventional method of coating includes applying a composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
The size of the treatment region within an anatomical lumen may vary. Multiple stents may be deployed adjacent to each other to treat relatively large regions of a vessel. However, positioning and deployment of multiple stents can be time consuming and may require a specialized delivery device capable of accommodating multiple stents. There are also issues associated with the regions where the stents meet. If the multiple stents are not abutted closely, there can be regions between the stents which are not treated. To avoid this, when serial stents are implanted, they are typically overlapped. Overlapping creates other issues. The overlapped stent regions are stiffer and allow for less natural movement of the vessel. They also have a double thickness of struts which must be endothelialized for complete healing and, in the case of drug eluting stents, they have double the load of drug and carrier polymer. There can be certain economies associated with using a single 2× length stent as apposed to two 1× length stents as the manufacturing cost of producing a 2× length stent is not twice the cost of producing two 1× length stents. For these many reasons, longer stents may be used, such as stents with an overall length greater than 30 mm. However, methods and devices for coating shorter stents may produce a greater incidence of coating defects in longer stents. Coating defects may include non-uniform surface characteristics, non-uniform thickness, bare spots, and flaking. FIGS. 13 and 14 show coating defects on the luminal or inner surface of a stent having a 4 mm overall diameter and 38 mm overall length. It is desirable to minimize coating defects for several reasons. Coating defects can serve as an initiation site for coating peeling or flaking that can create embolic debris. The rough surface generated, and stagnant regions of blood flow in the case of flaps or packets can serve as a nidus for thrombus formation. Furthermore, coating defects can lead to undue variation in the amount, concentration, and release rate of the drug from the stent coating.
Accordingly there is a continuing need for a coating method and system that minimizes stent coating defects, especially for longer stents.